Spin Transition Molecular Alloys:  An Attempt of Fine Tuning of the Transition Temperatures

The goal of this work has been to propose a novel approach to fine tune the transition temperatures of spin transition materials, based on the concept of spin transition molecular alloys. It has concerned the alloys of composition [Fe(R(1)trz)(3-3x),(R(2)trz)(3x)]A(2), where R(1)trz and R(2)trz stand for two different 4-substituted-1,2,4-triazole ligands, and A(-) stands for a counteranion. It has been shown that for any composition defined by x the alloy gives a unique spin transition regime, while the mixture (1-x)[Fe(R(1)trz)(3)]A(2) + x[Fe(R(2)trz)(3)]A(2) of the same chemical composition shows two spin transition regimes with the relative intensities 1-x and x, respectively. In the case of the alloys [Fe(Htrz)(3-3x)(NH(2)trz)(3x)](ClO4)(2) . H2O, the inversion temperatures T(1/2)up arrow and T(1/2)down arrow in the warming and cooling modes, respectively, have been found to vary almost linearly as a function of x. For x = 0.15, the alloy is bistable at room temperature. A theoretical treatment of this molecular alloy effect in the mean-field approximation has been proposed, which rationalizes the experimental findings. The advantages and drawbacks of this approach have been discussed in detail, and some possible applications have been presented.